Signals can be sent in frequency division duplex (FDD) mode as shown in FIG. 1, for example a plurality of voice signals (V1, V2, and V3) and a plurality of data signals (D1 and D2). The voice signals are transmitted typically at a lower power than the data signals, since the voice signals can be transmitted with a lower data rate (such as by a higher spreading factor) without a significant loss in signal quality. For example, a voice signal can be transmitted with a spreading factor of 64 (64 chips per data bit), whereas a data signal may be transmitted with a spreading factor of four chips per data bit, due to the higher transmission rate. As illustrated in FIG. 1, several voice and data signals can be transmitted in the same spectrum. For voice communications (V1, V2 and V3), the voice signals usually need a small amount of system bandwidth, and accordingly transmission power. For high rate data signals, a larger bandwidth is required which typically requires higher transmission power levels.
The uplink of the FDD universal mobile telecommunications system (UMTS) supports a potentially large number of simultaneously transmitted codes. The signature sequences of the codes are highly non-structured with long codes having a period of one frame (38,400 chips). Short signature sequences are permitted as an option; however, even these short sequences have a period of 256 chips. By comparison, in time division duplex (TDD) mode where multiuser detection techniques are more typically employed, the signature sequences are far shorter and more rigidly structured, with a period of 16 chips.
The lack of structure of the signature sequence in FDD combined with a large number of users that the receiver may be required to support makes it infeasible to implement standard multi-user detectors (MUDs), such as decorrelator and minimum mean square error (MMSE) type receivers in such systems. Other popular MUD receiver structures are not necessarily suitable here either. For example, successive interference cancellers (SICs) do not perform well with a large number of codes of approximately the same power. Parallel interference cancellers (PICs), are complex and do not necessarily deliver significant performance improvements because their effectiveness falls as the total interference rises. Accordingly, PICs tend to perform poorly for recovery of voice user data in the presence of several high data rate users.
Additionally, there is significant amount of data shuffling that occurs between the physical channel demodulation and the channel decoders. This makes joint channel demodulation and decoding techniques nearly infeasible.
Accordingly, it is desirable to have alternate MUD-type receiver designs for such systems.